This invention relates to improvements in methods and apparatus for the control of a suspended weight from a floating vessel and is particularly directed to the control of forces imposed on the running string of a floating vessel employed in drilling, coring, running casing, reaming, cementing, testing, or other services in bore holes drilled in submarine environments where the vessel is subjected to wave or tidal action.
The particular and preferred object of this invention is to improve the operation of such systems. As is well known in the petroleum industry, the string, due to the great length above the drill collars, is a very flexible member subjected to considerable stretch due to its length and due to its own weight. The weight on the bit is less than the total static weight of the drill string imposed by the drilling lines as is well understood by those skilled in this art. The practice during drilling is to keep the drill pipe above the drill collars in tension. The drill collars act as a weight-producing element which exerts the load on the drill bit.
In certain of the systems of the prior art, this is accomplished by a gas pressure in an accumulator which pressurizes liquid in a hydraulic cylinder underneath the piston which supports the weight. The cylinder may be positioned to support a crown block or connected to the traveling block conventional in drilling derricks.
Wave action imposes a vertical oscillatory motion on the vessel which is imposed on a hydraulic cylinder resulting in variations in the tension in the drill pipe and perhaps in variation in the load imposed upon the drill bit, when this is employed or any other load connected to the piston rod. In the case of the hydraulic-pneumatic systems or pneumatic systems, the pressure on the liquid underneath the piston rod is maintained by gas pressure in an accumulator; such systems are shown in the Hanes et al. U.S. Pat. No. 3,714,995 and the Larralde et al. U.S. Pat. No. 3,718,316, and in U.S. Pat. No. 3,847,607.
Experience with such systems has shown that a variation in the lifting force of about .+-. 2% to .+-. 5% of the suspended weight may be experienced at each wave cycle under not unusual conditions, even when no drilling advance is maintained. With drill advancing during drilling, an additional force variation may occur. The results of these motions are that the pressure in the cylinder fluctuates and the degree of fluctuation increases as the drilling progresses.
In systems in which the suspended weight is supported in a pneumatic system which does not employ a liquid pressure transfer medium as in the former case, like load variation occurs.
Floating vessels operating as drilling vessels in the open sea may experience vertical motions, i.e., heave due to wave action ranging, for example, from as low as 2 inches to 20 feet or more trough to crest as, for example, has been experienced in the drilling of the North Sea. However, under ordinary conditions, the ships are on station and drilling when the heave is not more than about 10 to 15 feet. The wave action imposes a vertical displacement of the drilling vessel which in practical effect is sinusoidal. The period of such cycles has been reported in the range of 8 to 16 seconds but may be either greater or less.
The demand on the string will vary, depending on the services which they are to provide. Thus, for landing casing or instrument survey, it is desirable to hold the piston fixed in space at the desired level in order that the casing or instrument is not subject to displacement.
There is also another circumstance where it may become important that the piston be maintained at relatively stationary position in space. Thus, when the well suddenly develops a high pressure due to the production of gas and it becomes necessary to close the blow-out preventer, it becomes highly important that the drill string remain fixed in space and not oscillate in the closed blow-out preventer so as not to damage it.
During drilling, however, the drill is to be advanced at a controlled rate while maintaining a controlled weight on the bit. It is desirable to maintain a desired upper load limit on the bit in order that excessive stresses and torque are not developed which will be so large as to injure or even cause rupture of the drill pipe. On the other hand, it is desirable that the load on the bit be not reduced excessively so that the rate of advance is unreasonably reduced. Since the cost of operation of the drilling operation is materially affected by the drilling rate, it is desirable that the drilling rate be maintained at as high a rate as is consistent with safety. This is established by the driller based on his experience and the performance of the drilling operation under consideration. The driller sets the load required for the drilling advance to make the advance be at a satisfactory rate consistent with safety.
In the pneumatic systems, with or without a hydraulic transfer, during the heave cycle as the volume V.sub.0 of the gas, effective at a pressure p.sub.0, as the force-producing medium, varies as a function of the vertical motion of the cylinder, the resultant pressure p.sub.1 due to a change in volume .DELTA.V.sub.0, is given by the gas law. ##EQU1##
The power k is the ratio of the specific heat of the gas at constant pressure divided by the specific heat at constant volume. For practical purposes, experience with such system has shown that it may be taken as Unity, since for practical purposes the system may be assumed to be operating under isothermal conditions. If temperature varies significantly, this variation may be taken into consideration by suitable modification of the value of "k." For purposes of illustrating the principles of my invention, "k" may be taken as having the value of unity. In going from the midpoint of the heave to the crest of the wave, this quarter of the cycle is termed the first quarter. The value .DELTA.V.sub.0 is negative in the first and last quarter of each cycle and positive in the second and third as per above. ##EQU2##
In the second quarter the attained pressure is: ##EQU3##
In the third quarter the attained pressure is: ##EQU4##
In the fourth quarter the pressure attained is: ##EQU5##
It will be seen that the variation in the load, i.e., changes in the pressure during the heave, is diminished by making the volume V.sub.0 sufficiently large. The value of p.sub.0 is fixed by the design dimensions of the effective area of the piston and by the forces desired to be established. In practice these parameters are dictated by practical requirements. For practical purposes the volume of the gas has an upper limit and the variation in pressure will depend on the nature of the wave action.
Another limitation that such systems have arises from the fact that if the drill is to advance as in drilling the value of .DELTA.V.sub.0 is changed by the volume change resulting from the advance during drilling.
The progressive increase in pressure during drilling results from the progressive decrease in volume resulting from the motion of the piston in the cylinder during the descent of the piston. The volume change in "n" cycles is symbolized by n.DELTA.v.
In the first quarter of any succeeding cycle the attained pressure is: ##EQU6##
In the second quarter, the attained pressure is: ##EQU7##
In the third quarter, the attained pressure is: ##EQU8##
In the fourth quarter, the attained pressure is: ##EQU9##
In such an operation, this variation repeats in each cycle of the wave.
The resultant variation in lifting force will, depending on the modulus of the running string, result in a cyclic variation in the stretch of the string. It may also result in a variation in the net weight imposed on the bit depending on the drag effect of the mud in the hole employed in drilling. Variation in drill bit loads results in impact loads which have a serious effect in reducing the "life" of the bit and to increase the frequency of "round trips" which increases the cost of drilling.
The value of (n) must be limited not only by the dimensions of the cylinder, but also so that the retained pressures do not become excessive. In order that drilling continue beyond the point, the system, including the cylinder and the piston, must be lowered. This may be done by adjusting the drilling lines so as to lower the cylinder and thus the running string to accommodate the advance.